The invention pertains to polymerization reactions, particularly to monitor and control the rate and the amount of conversion in such reactions. More particularly, the invention pertains to accurately determining the rate and amount of conversion at a particular moment in a polymerization reaction so as to control the rate of conversion and to optimize cooling resources.
Most polymerization reactions today are run open loop with respect to the product quality (end-use) properties. Also operations involved in the manufacturing process are scheduled by a simple timer without attention to the actual progress of reaction.
In the last decade, the affordability of powerful computers finally made it possible to exploit the advanced control concepts control theorists have been developing since the 1960""s. As a result, control of continuous processes like refinery distillation columns or power generation units has seen a rapid evolution from single loop proportional, integral and differential (PID) controllers to multivariable predictive controllers with built-in constraint optimization whose performance cannot be matched by the old PID solutions.
For a number of reasons, this progress so far has avoided batch processes. Control wise, most batches are still run the way they were thirty or more years ago. If there was a change, it affected control hardware, but not control algorithms. A batch recipe still prescribes time profiles of temperatures or pressures to be followed by a batch reactor in order to make the product. Feedback controllers, usually PID""s, are routinely used to make the batch track the recipe in the presence of variations in feedstock concentration and purity, catalyst activity, reactor fouling and so on.
Maintaining batch recipe temperatures and pressures is important but it should not be the control objective. After all, the process owner does not sell batch temperatures or pressures. They are mere process parameters and, by themselves, are not even sufficient ones. It is well known and exemplified below for the case of polymerization processes, that two batches with perfectly identical temperature and pressure profiles can still have different rates at which monomer is converted into polymer, and thus yield products with inconsistent quality. When comes to the end-use parameters of the real product, which are determining its marketable quality, most batch processes are still run open loop, with all the negative consequences that an open loop recipe execution entails.
With the present invention, that approach is replaced with a feedback controller for polymerization processes that closes the loop using a measurement directly tied to the product""s marketable quality, and thus employs feedback to eliminate quality variations and inconsistencies due to the fluctuations of process inputs and operating conditions.
The invention is a polymerization control that allows the user to specify independently the reaction mixture temperature and the degree of monomer conversion profiles as a function of time, and execute them under feedback control. This both improves the run-to-run consistency of the product and reduces the uncertainty of the reaction time and coolant consumption at any given instant. Because the coolant availability often is the limiting factor of production capacity, the improved predictability of individual batch runs offers an opportunity to improve batch planning and scheduling and thus increase the plant yield without expensive retrofits.
This invention enables the controller to employ feedback for the control of product properties without the need for specialty sensors to measure the properties and run the polymerization process on the basis of its inner time reflecting its actual progress. As a result, the invention makes it possible, first, to manufacture polymers with consistent quality and, second, to improve process yield by allowing for better utilization of the available cooling capacity without sacrificing process safety.
The invention includes an inferential sensor, whose concept is based on the observation that for polymerization processes, in which heat is released by a single reaction, the amount of heat released is proportional, albeit in a nonlinear way, to the degree of the monomer conversion. Hence, by carefully calculating the reactor""s thermal balance on-line one can continuously infer the degree of conversion and use it for control. Once the actual degree of conversion can be determined and ultimately controlled, one can also control the cooling duty of the reactor and thus make it conform with the cooling capacity allotted to it by the plant scheduler.
Superficially, an advanced batch control system utilizing the inferential sensor looks very much the same as a conventional one. In both cases, measurements of temperatures and flows of the reactor coolant as it enters and leaves the reactor jacket or cooling coil will constitute the bulk of input data. In addition to that data, the inferential sensor may require additional data reporting temperatures at some other reactor spots and on the amounts and temperatures of feedstocks and catalysts. If some data on their composition are available, they can also be used with advantage for a more accurate inference.
The significant difference is in what the controllers do internally with the data. In a conventional batch controller, the data are used directly to control the reactor mixture temperature by manipulating the incoming coolant flow and temperature. In an advanced controller, the data are fed into the inferential sensor instead, where they are used to infer the current value of the degree of monomer conversion. This quantity is then passed to the controller part of the advanced batch controller.
Even though the inferential sensor could be implemented as a stand-alone device and thus resemble physical sensors, this option is unlikely. The reason is that the sensor involves a nonlinear dynamic model of both the process and the reactor, whose state must be kept in sync or coordinated with reality using a state estimation algorithm driven by the measured temperatures and flows. Once the model is available, it is shared with the advantage of a model-based (nonlinear) controller.
Polymerization reactions are exothermic (i.e., a chemical change in which there is a liberation of heat, such as combustion). The overall amount of heat released by a reaction from its start up to a given instant depends on how much of the monomer(s) has been converted into polymer. This measure of released heat indicating the degree of monomer conversion is a more reliable indicator of reaction progress than physical time because the same reaction can be running slower or faster depending on the initiator (i.e., catalyst) activity, reactant purity and other effects that may be difficult to measure directly. Moreover, for many polymerization reactions the degree of conversion is linked to the product quality and thus can be used for closing the loop for the product quality feedback control in place of specialty sensors.
The degree of conversion is not measured directly, but the invention involves inferring its running value by dynamically evaluating the reactor heat balance. This invention involves four concepts. First, there is the way of inferring the degree of conversion from the dynamic evaluation of the reactor heat balance. Second, the use of the degree of conversion replaces specialty sensors for feedback control with respect to the product quality (end-use) properties. Third, the use of the degree of conversion replaces physical time for the timing of process related operations like valve opening and closing, controlling the heat supply/removal, dosing the reactants, and so forth. Fourth, the sensor allows an accurate prediction of the batch evolution and thus makes it possible to accurately predict the cooling need profile from the current instant. out to the batch termination.
In this invention, the reaction mixture temperature and the integral heat rate are treated as two independent process variables. This approach provides the user the freedom to specify batch recipes in a way that defines the evolutions of either variable during the batch run, and to execute them under tight, high performance control. Because the degree of monomer conversion is proportional to the integral heat rate for many important polymers including PVC, controlling the two variables gives the user independent control over two basic determinants of product quality. Even more importantly, such control fully defines the heat release at every instant of the batch run, thus making it possible to better utilize the available cooling capacity through more reliable planning and scheduling. To control the temperature and integral heat rate independently, the proposed method manipulates the amount of heat added to or taken out of the reaction and the amounts of the initiator(s) and inhibitor added during the batch run.
The present invention improves the yield of a PVC or polymerization manufacturing plant in two ways. First, this approach provides better feedback control of individual reactors, thus reducing the uncertainties of the reaction time and coolant consumption at any given instant. Because the benefit of plantwide planning and scheduling is dependent on the quality of predictions that were used for the plan and schedule development, better reactor control is a technological enabler of better planning and scheduling. Specifically, more reliable predictions of the coolant consumption allow the planner to run the plant with smaller cooling capacity margins without sacrificing the plant safety, thus increasing the plant yield. The controller of this invention can accelerate or decelerate the reaction without changing the reaction mixture temperature. Consequently, it can reduce the reaction time without sacrificing the product quality by taking advantage of any available cooling capacity. Second, this approach improves run-to-run product consistency and allows one to tighten the product specifications, which also adds to the yield increase, by reducing the off-specification production.
It is well known that some polymers could be produced by reactions running at greater speeds without any significant degradation of their quality, if only the reactors used could handle the increased heat flow. A good example is the manufacturing of PVC by the suspension process. A PVC plant in Canada uses water from a river as a coolant for its reactors. In the winter, when the water temperature is about 0.6 degrees Celsius (i.e., 33 degrees Fahrenheit), a batch takes about 5 hours to complete. However, in the summer, when the river water temperature raises to 22 degrees C. (72 degrees F.), the same batch, with comparable product quality, takes 8 to 9 hours, because the drop in the available cooling capacity forces the plant operator to slow down the reaction rate by using smaller amounts of the initiator.
In the above example, the coolant""s supply is unlimited and the restriction comes from its increased temperature and limited water circulation flow through the reactors"" jackets. Another example is a PVC plant that uses chilled water as the coolant for its jacketed reactors. The plant has a centralized utility which supplies water to a dozen or so reactors. Because the chilled water is expensive and its supply is limited, water exiting the reactors is partly recycled by mixing it with the freshly chilled water coming directly from the cooling towers. This creates a variable production environment, wherein the availability of the chilled water depends on the number of batches currently in progress as some reactors are always being charged or discharged, while others are temporarily out of service for cleaning and maintenance. Also, the chilled water temperature may fluctuate with the weather and the time of the day.
Before starting a batch, the operator must make a decision on how fast he can afford to run it without risking a temperature runaway and choose the appropriate amount of the initiator(s) which is then added to the reactor charge. To some degree, this decision is guesswork as the operator has to consider the effects of gradual deposit buildup on the reactor walls on heat removal. Once the reaction gets going, the operator can, in principle, speed it up or slow it down manually by adding the initiator or inhibitor, respectively, but this is not normally done. Once started, the batch is run open loop without further operator interference until its completion, which is indicated by the pressure drop in the reactor.
Given the uncertainty concerning the cooling capacity that will be actually available during the upcoming batch run and the impossibility to exactly determine the initiator dosing beforehand and to correct it later, the operator has to play it safe and make decisions that on the average might be overly conservative. This cuts into the reactor yield. Obviously, a better control over the actual rates of individual reactors in the plant would make it possible to reduce the current technological margins without endangering plant safety and thus create an opportunity to employ tighter plantwide optimization.
If one had better control over the reaction progress, then one could even think about more unusual ways to increase the plant yield. Currently, for each reaction the operator defines its speed before it begins by dumping a particular amount of the initiator(s) into the mixture. But there might be a window of opportunity when a large amount of chilled water is available, say, for two hours, because a couple of other reactors happened to finish simultaneously and have to be discharged and recharged. A batch controller that would allow the operator to temporarily accelerate the running reactions for the two hours to take advantage of the unexpectedly available cooling capacity and then bring them back to the original rate by applying a suitable amount of an inhibitor, without disturbing the reaction mixture temperature, would further improve the plant yield.
Making the batch follow a given temperature and conversion rate profiles not only improves the quality and run-to-run consistency of its product but, perhaps even more importantly, enables one to make accurate predictions of the heat release during the batch run. As a result, one can make reasonable and justifiable provisions for the expected cooling duty needed to keep the mixture at the desired temperature all the way up to the reaction end, and thus better utilize available plant resources through more reliable plantwide planning and scheduling.
The present advanced batch control, along with the follow-up plantwide optimization it enables, may have a major economic impact on plant performance. Consider, for example, a plant with fifteen reactors running so that the cooling capacity reserve is 10 percent. Since the cooling availability is the limiting factor, reducing the reserve to four percent would increase its output by six percent, which is almost tantamount to adding another reactor to the plant, without the expense of its construction and maintenance.